I have 4 albino cories in my 20 gallon long tank. I know cories do best in sand, so I got some black aquarium sand from petco. When I put it in, it raised the ph to over 9 and I lost half my fish. The other cories are barely alive and hardly eat or move. I am going to move them into a 5 gallon tub while I fix the ph.
What substrate should I get for the cories? I really don't want to risk getting sand again because there's a good chance it will mess with the ph. Also, pool filter sand only comes in white which would blend in with my cories and I really don't want to spend another $40+ on petstore sand only to have it fail again. Would cories be okay in black gravel? I know it can be hard on their barbels, but if it was smooth and pretty small, do you think it would work?

I know it seems unlikely that it was the sand because so many people use that brand without a problem. However, I put some in a bucket and over the course of 2 days, it raised the ph from 7.5 to 8 and I'm sure it would have gone higher if I left it longer. I rinsed out the sand and repeated the test again to make sure but the same thing happened so I'm positive it was the sand. Do you think it's possible I got a bad bag?

What kind of gravel is it? Did you get it at Petco too?
Also, do you think I should try using things like wood to lower my ph or should I just get rid of the sand altogether

My ph is between 8 and 9 and my fish are fine but all I have is bettas, cories and otos (and snails). I haven't check ph since adding the wood, I should do that right now........

OK it's still darker than the 7.6 color on the card.

The gravel I got at a LPS and you can tell by looking at it that it's real little smooth-ish rocks, not the rough colored gravel people usually get at the store. I also bought several bags of smooth little glass pebbles (like for flower arrangements) at a craft store and mixed those in. I will add on a photo of my fish where you can see the gravel kind of close up. The green and white ones are the glass: